This invention relates to an exposure control method for controlling the amount of exposure of a photosensitive substrate in an exposure apparatus used in a lithographic process for manufacturing, e.g., a semiconductor element, a liquid crystal element, an image sensing device (CCD, etc.) and a thin magnetic head. The method is applicable not only to a batch exposure type exposure apparatus but also in a case where the amount of exposure is controlled using a step-and-scan scanning exposure type projection exposure apparatus in which part of the pattern on a mask is projected onto a photosensitive substrate and the mask and substrate are then scanned synchronously with respect to a projection optics unit, whereby the mask pattern is transferred to shot areas on the substrate to expose these areas to the mask pattern. The invention further relates to an exposure control apparatus and device manufacturing method to which this exposure control method is applied.
FIG. 2 illustrates a projection exposure apparatus according to the prior art. The apparatus includes a light source 1 such as a high-voltage mercury-vapor lamp which emits illuminating light. The light from the light source 1 is condensed to a point by a condensing mirror 2 and impinges upon a fly-eye lens 4 through an optics unit 3. There are instances where a laser or the like may be used as the source of illuminating light, in which case the condensing mirror 2 is unnecessary and the light from the laser need only impinge upon the fly-eye lens 4 through the optics unit 3.
The fly-eye lens 4 is a bundle of rod lenses the entrance and exit surfaces of which have their focal points on each other""s surface. A group of light beams that impinge upon the rod lenses at an identical angle are condensed at the exit surfaces and form a number of points of condensed light on the exit surface of the fly-eye lens.
Utilizing the group of condensed points of light formed on the exit surface of the fly-eye lens, the optics unit 5 uniformly illuminates the position of a diaphragm 6, which controls an illuminated area at a position that is conjugate with the plane of a mask 8. An optics unit 7 is for forming the image of the position of the uniformly illuminated diaphragm 6 on the mask surface 8. Uniform illumination of the mask surface 8 is achieved by forming the image of the position of the uniformly illuminated diaphragm 6 on the mask surface 8. It should be noted that the position of mask 8, the position of diaphragm 6 and the entrance surface of the fly-eye lens 4 are located at conjugate points.
The apparatus further includes a projection optics unit 9 for forming the image of the pattern of mask 8 on a substrate 11. A photosensitive agent that has been applied to the substrate 11 is exposed to the mask pattern by the illuminating light from the illuminating optics unit. The projection optics unit 9 is a telecentric unit in which projection magnification does not change even if the position of the mask 8 or the position of the substrate 11 shifts along the optical axis. The arrangement is such that a principal ray which passes through the center of the projection unit at the position of a diaphragm 10 perpendicularly intersects the mark surface and the substrate.
It should be noted that the diaphragm 10 of the projection optics unit 9 and the exit surface of the fly-eye lens 4 are located at conjugate points.
The apparatus further includes a movable stage 12 on which the substrate 11 and an exposure sensor 15 are mounted. The exposure sensor 15 can be moved over the illuminated area when the amount of exposure at a position identical with that of the substrate 11 is measured with stepping movement for exposing a plurality of shots on the substrate 11.
In such a projection exposure apparatus used in the manufacture of semiconductor devices and the like, it is required that the substrate be subjected to a proper amount of exposure, which depends upon the photosensitive agent that has been applied to the substrate, in order that the mask pattern will be transferred to the substrate in an optimum fashion. If the amount of exposure is less than the proper amount in a case where a positive pattern and a negative resist are used, for example, the photosensitive agent will not be sensitized sufficiently and the lines of the pattern may become too fine and be rendered discontinuous at points.
If the amount of exposure is too large, on the other hand, the photosensitive agent will be sensitized excessively and the lines of the pattern may become so thick that neighboring lines will contact each other.
Further, if the amount of exposure is less than the proper amount in a case where a negative pattern and a positive resist are used, the photosensitive agent will not be sensitized sufficiently and the lines of the pattern may become so thick that neighboring lines will contact each other.
If the amount of exposure is too large, on the other hand, the photosensitive agent will be sensitized excessively and the lines of the pattern may become too fine and be rendered discontinuous at points. In any case, when exposure is carried out with an improper amount of exposure, a suitable pattern cannot be formed on the substrate. This invites a decline in yield when semiconductor devices or the like are manufactured.
Control of the amount of exposure to which a substrate is subjected must be controlled in order to obtain the proper amount of exposure. However, the amount of exposure being applied to a substrate cannot be measured directly during the transfer of the mask pattern to the substrate. If the amount of exposure is measured along the optical path of the exposing light, the shadow of the exposure sensor has an influence when the mask pattern is transferred to the substrate. For this reason, the amount of exposure is controlled upon measuring the amount of exposure at a position which is at a conjugate point with the substrate and offset from the optical path of the exposing light.
More specifically, use is made of a half-mirror 13, which has a very low reflectivity, inserted into the optical path of the exposing light in order to produce a position which is at a conjugate point with respect to the substrate 11 and offset from the optical path of the exposing light. That is, the half-mirror 13 produces a position which is at a point conjugate with the substrate 11 and offset from the optical path of the exposing light at the position of an exposure sensor 14. The sensor 14 is placed directly in front of the point conjugate with the substrate 11 and at an inclination relative to the optical axis of the exposing light for the purpose of measuring the amount of exposure from the light diverted to it by the mirror 13.
The exposure sensor 14 is so adapted as to be capable of measuring an amount of exposure that corresponds to the amount of exposure exactly at the center of the illuminated area, namely at the position of the substrate 11 on the optical axis. Before the substrate is exposed, the exposure sensor 15 mounted on the stage is moved to the center of the illuminated zone, trial exposure is carried out and the relationship between the amount of exposure at the position measured by the exposure sensor 14 and the amount of exposure on the substrate is found, thereby making it possible to estimate the amount of exposure on the substrate from the output of the exposure sensor 14.
A controller 16 is provided for controlling the amount of exposure. On the basis of the output of the exposure sensor 14, and in accordance with a predetermined control program, the controller 16 controls the amount of exposure by controlling the opening and closing of a shutter 17, the transmittance of beam attenuating means 18, the transmittance of which is variable, and the input to the light source 1.
In accordance with the above-described prior art, however, in order to measure the amount of exposure at a position that is conjugate with the substrate 11 along the optical axis, it is required that the half-mirror 13 be inserted deeply in such a manner that the position conjugate with the substrate 11 along the optical axis will be offset from the optical path, as illustrated in FIG. 2. Further, in order to assure that the optical path of the reflected light from the half-mirror 13 to the exposure sensor 14 will not be obstructed by the lens 5 immediately in front of the half-mirror 13, the half-mirror 13 and the lens 5 directly in front of it must be spaced apart.
By way of example, FIG. 5 illustrates the optical path from the lens 5 directly in from the half-mirror 13 to the diaphragm 6 controlling the illuminated area in a case where the inclination of the half-mirror 13 is such that the half-mirror is almost perpendicular to the optical axis. In this case, length A which the half-mirror 13 occupies along the optical axis can be reduced, as is obvious from FIG. 5. However, in order to assure that a reflected light beam will not be obstructed by the lens 5 immediately in front of the half-mirror 13, it is required that the relation C less than Btanxcex8 hold, where C represents the radius of the lens 5 and xcex8 represents the angle defined by the reflected light beam and the optical axis. As a consequence, the angel xcex8 becomes small and spacing B between the half-mirror 13 and the lens 5 directly in front of the half-mirror 13 becomes too large.
By contrast, FIG. 6 illustrates the optical path from the lens directly in from the half-mirror 13 to the diaphragm 6 controlling the illuminated area in a case where the inclination of the half mirror 13 is such that the half-mirror is nearly parallel to the optical axis. In this case, the angle xcex8 between the reflected light beam and the optical axis can be enlarged, thereby making it possible to shorten the distance between the half-mirror 13 and the lens 5 directly in front of it, as is evident from FIG. 6. However, the length A which the half-mirror 13 occupies along the optical axis becomes too large.
The end result is that with the prior art described above, a length A+B along the optical axis necessary to measure the amount of exposure by diverting light using the half-mirror 13 cannot be made shorter than a specific length, with the consequence that the space occupied by the exposure measuring means cannot be reduced. However, as a result of the improved performance and capabilities sought for projection exposure apparatus, the optics unit employed in such apparatus has become extremely complicated and there is a tendency for such apparatus to be of ever increasing size. Accordingly, in order to reduce the size of a projection exposure apparatus even marginally, there is strong demand to reduce the space occupied by the exposure measurement means as much as possible.
The present invention has been proposed to solve the problems of the prior art and its object is to provide an exposure control method which uses exposure measuring means that occupies little space, as well as a projection exposure apparatus and device manufacturing method which employ this method.
According to the present invention, the foregoing object is attained by providing a method of controlling an amount of exposure in which when a pattern on a reticle is illuminated by illuminating light from a light source so as to be projected onto a substrate to expose the same, an amount of exposure at a position substantially conjugate with an illuminated area on the substrate is measured and the amount of exposure applied to the substrate is controlled based upon a result of the measurement, wherein the position at which the amount of exposure is measured is a position substantially conjugate with an off-optical axis position in the illuminated area on the substrate.
In accordance with the prior art, the amount of exposure of the substrate is controlled upon measuring the amount of exposure at a position, which is along the optical axis, substantially conjugate with the substrate. By contrast, in accordance with the present invention, the amount of exposure of the substrate is controlled upon measuring the amount of exposure at a position, which is offset from the optical axis, substantially conjugate with the substrate. As a result, the half-mirror for measuring the amount of exposure is reduced in size or eliminated, thereby achieving a reduction in the space occupied by the means for measuring the amount of exposure.
The present invention further provides an exposure apparatus having projection exposure means for illuminating a pattern on a reticle by illuminating light from a light source so as to project the pattern onto a substrate to expose the same, exposure measurement means for measuring an amount of exposure at a position substantially conjugate with an illuminated area on the substrate, and the exposure control means for controlling the amount of exposure of the substrate based upon the result of the measurement, wherein the exposure measurement means measures the amount of exposure at a position, which is offset from the optical axis, substantially conjugate with the substrate and the exposure control means controls the amount of exposure of the substrate.
The present invention further provides a device manufacturing method for manufacturing a device by illuminating a pattern on a reticle by illuminating light so as to project the pattern upon a substrate to expose the same, wherein when exposure by projection of the illuminating light is performed, an amount of exposure at a position substantially conjugate with an illuminated area on the substrate is measured and an amount of exposure of the substrate is controlled based upon the result of the measurement, wherein the amount of exposure of the substrate is controlled upon measuring the amount of exposure at a position, which is offset from the optical axis, substantially conjugate with the substrate.